Japanese Patent Application Laid-Open Publication No. H07-74328 (Patent Document 1) discloses a technique in which a first semiconductor substrate to be an SOI layer and a second semiconductor substrate to be a support substrate are brought into close contact with each other in a clean atmosphere and are thus directly joined together. Warpage of the SOI substrate can be reduced by forming a trench in an oxide film of the first semiconductor substrate to be the SOI layer.
Japanese Patent Application Laid-Open Publication No. 2000-31252 (Patent Document 2) discloses a technique in which a thickness of an oxide film formed on a rear surface of a wafer is detected prior to processing the wafer and a series of voltage sequences (separation sequences) is applied in accordance with the thickness. The wafer is smoothly separated by the application of the separation sequences.